Molecular imaging can be broadly defined as the characterization and measurement of biological processes at the cellular and molecular level in mammals and human patients. In contradistinction to “classical” diagnostic imaging, for example, magnetic resonance (MR), computed tomography (CT), and ultrasound (US) imaging, molecular imaging analyzes molecular abnormalities that are the basis of disease, rather than imaging the end-effects of these molecular alterations. Specific imaging of molecular targets allows earlier detection and characterization of disease, as well as earlier and direct molecular assessment of treatment efficacy. Molecular imaging can theoretically be performed with different imaging technologies, up to now preferably with nuclear imaging technologies, (e.g., PET and SPECT imaging) which have high sensitivity of probe detection. The IV administered imaging probes typically recognize a given target. Alternatively, some probes detectable by MR imaging have been developed (Moats et al., Angewandte Chemie Int. Ed., 36:726-731, 1997; Weissleder et al., Nat. Med., 6:351-5, 2000), although their detection threshold is generally in the micromolar instead of the pico/femtomolar range of isotope probes.
An alternative molecular imaging method is to use fluorescent probes for target recognition. For example, enzyme activatable fluorochrome probes are described in Weissleder et al., U.S. Pat. No. 6,083,486, and fluorescent molecular beacons that become fluorescent after DNA hybridization are described in Tyagi et al. (Nat. Biotechnol., 16:49-53, 1998). Fluorescent activatable probes have been used to label specific tissue for in vitro culture and histologic sections and are detected using fluorescence microscopy. When administered in vivo, fluorescent activatable probes have been detected by surface-weighted reflectance imaging (Weissleder et al., Nat. Biotechnol., 17:375-8, 1999); (Mahmood et al., Radiology, 213:866-70, 1999).
A need exists to be able to detect molecular imaging probes at or near the surface of exposed tissue during surgery, without confocal microscopy and related limitations on the field of view and depth of focus. Such methods and systems could be used in clinical settings to guide surgery or therapy when localization of the target is important for treatment, such as for example in cancer treatment. The ability to detect single cells marked by probes would guide the surgery of tumor removal. A surgeon could scan the resection area to determine if all of the cancer has been removed during the surgery which could provide a level of effectiveness that could not otherwise be achieved. Should the surgeon remove all cancerous cells upon the initial surgery, further cancer recurrence could be mitigated or avoided and adjuvant treatments could be reduced or eliminated (e.g. followup surgeries, radiation therapy, chemotherapy, etc.) resulting in reduced patient discomfort, morbidity, and significant cost savings to the healthcare system.